arthropods

arthropods The arthropods are a very large phylum of organisms that are characterized by the presence of a segmented body with a hard exoskeleton and many jointed legs (the name ‘Arthropoda’ means ‘jointed legs’). More species of arthropods have been described than all other kinds of animals put together: almost three-quarters of the million or so known animal species are arthropods. The major groups of modern arthropods are the crustaceans (lobsters, crabs, barnacles, and ostracods) the centipedes, the millipedes, the arachnids (spiders and scorpions), and the insects, which are by far the largest class of all, totalling almost 500 000 species. Important fossil groups are the trilobites, a large group of Palaeozoic marine arthropods of uncertain affinity, and the eurypterids, Palaeozoic marine forms related to the arachnids. It has been suggested that the arthropod body plan might be polyphyletic (i.e. have developed several times), and four phyla are thus sometimes designated: Trilobita, Crustacea, Chelicerata (eurypterids and arachnids), and Uniramia (insects, millipedes, and centipedes).

Because of the adaptability of their body plan, the arthropods have been important and successful organisms since their first appearance in the Early Cambrian. Although they originated in the ocean they have since moved into fresh water, and they were also the first animals to move on to the land and into the air. Despite this, the only groups that are used extensively in geology are the trilobites, which are important biostratigraphic indicators in the Palaeozoic, and the ostracods, which are used throughout the Phanerozoic.

David K. Elliott